Process for producing a ferritic stainless steel having an improved corrosion resistance, especially resistance to intergranular and pitting corrosion

ABSTRACT

The present invention relates to a process for producing a ferritic stainless steel having an improved corrosion resistance, and especially resistance to intergranular corrosion and to pitting corrosion. The steel is subjected, in a first phase, to cooling at a rate of between 400° C. and 600° C./hour down to a temperature of 900° C. and then, in a second phase, to rapid cooling at a rate of between 1200° C. and 1400° C./h.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for manufacturing a ferriticsteel having an improved corrosion resistance, and especially resistanceto intergranular and pitting corrosion.

2. Discussion of the Background

Japanese Patent No. 62,250,150 (Nippon Kokan) discloses acorrosion-resistant ferritic stainless steel whose composition is asfollows: carbon less than 0.04%, silicon less than 1%, manganese lessthan 1%, nickel less than 6%, chromium between 19 and 28%, molybdenumbetween 1 and 6%, nitrogen less than 0.03%, phosphorus less than 0.06%and sulfur less than 0.03%. This steel may also contain niobium and/ortitanium.

That document presents a ferritic steel having a high corrosionresistance, used for withstanding a mixture of phosphoric acid, ofsulfuric acid and of chlorine and fluorine ions.

Without a specific conversion, this steel remains difficult to produce.In addition, it is known that steels resistant to acid media are steelscontaining, in their composition, a relatively large amount of nickel.

Also known is Patent DE 3,221,087 (Thyssen) relating to the manufactureof a so-called superferritic CrMoNi stainless steel which includesconventional oxygen refining using an AOD or VOD process, continuouscasting of billets or slabs, optional intermediate cooling, andannealing followed by conversion into blooms and end- or semi-finishedproducts. The superferritic stainless steel has the followingcomposition: carbon between 0.01 and 0.05%, silicon less than 2%,manganese less than 1%, nickel between 1 and 4%, chromium between 21 and31%, molybdenum between 1.5 and 3.5%, nitrogen between 0.01 and 0.08%,phosphorus less than 0.0025%, sulfur less than 0.01%, titanium less than0.24%, zirconium between 0.005 and 0.5%, aluminum between 0.002 and0.12%, niobium between 0.1 and 0.6% and copper less than 3%. This steelmay also contain calcium, magnesium, cerium and boron, and the elementsof the composition satisfy the following relationships:

% Cr+10×(% Mo)+6×(% Si) lying between 48 and 58;

% Nb+% Zr+3.5×(% Al+2×% Ti) lying between 8 and 16×(% C+% N)

In this document, it is specified that some of the aluminum may bereplaced by doubling the amount of titanium on condition that there isat least 0.002% of aluminum. The steel is preferably hot rolled orforged directly after continuous casting, without intermediate cooling.

OBJECTS OF THE INVENTION

One object of the invention is to improve the corrosion resistance of aferritic steel, especially the resistance to intergranular and pittingcorrosion, while at the same time maintaining a conversion processcompatible with the conversions of common so-called 17% chromiumferritic steels.

DETAILED DESCRIPTION OF THE INVENTION

One subject of the invention is a process for producing a ferriticstainless steel having an improved corrosion resistance, and especiallyresistance to intergranular corrosion and to pitting corrosion. Thesteel so produced is part of the invention. In the invention processsteel, preferably in slab form, containing in its composition (by weightbased on total weight):

18% <chromium<27%

1% <molybdenum<3%

1% <nickel<3%

manganese<1%

silicon<1%

carbon<0.030%

nitrogen<0.030%

0.075% <titanium<0.20%

0.20% <niobium<0.50%

sulfur<0.01%

phosphorus<0.1%

the balance being mostly or wholly iron and impurities resulting fromthe smelting of the materials necessary for the production, issubjected, in a first phase, to cooling from, preferably, above 950° C.,more preferably 1200°-1300° C., at a rate of between 400° C. and 600°C./hour down to a temperature of from 950°-850° C., preferably 900° C.,and then, in a second phase, to more rapid cooling at a rate of between1200° C. and 1400° C./h to a temperature of from 650°-550° C. All valuesbetween all given temperature and cooling rate ranges provided hereinare included as part of the invention as are all subranges therebetween.For example, cooling rates of 450°, 500° and 550° C./hour may be used inthe first phase, and cooling rates of 1250°, 1300° and 1350° C./hour maybe used in the second phase.

The other characteristics of the invention which may be present singlyor in combinations of two or more are:

after hot rolling, the strip obtained is subjected to rapid cooling andthen coiled at a temperature of less than 600° C. and preferably at atemperature close to (±10%) 550° C.

preferably, the steel, in slab form, contains in its composition byweight:

22% <chromium<27%

1% <molybdenum<3%

1% <nickel<3%

manganese<1%

silicon<1%

carbon<0.030%

nitrogen<0.030%

0.075% <titanium<0.20%

0.20% <niobium<0.50%

sulfur<0.01%

phosphorous<0.1%

the steel furthermore contains, in its composition by weight, less than0.20% of copper.

the elements of the composition of the steel furthermore satisfy thefollowing relationship:

0.07% <ΔNb=% Nb+7/4% Ti-7(% C+% N)<0.4%.

The invention also relates to a ferritic stainless steel obtained by theabove process and having improved corrosion resistance, especiallyresistance to intergranular and pitting corrosion, defined in itscomposition by weight based on total weight:

18% <chromium<27%

1% <molybdenum<3%

1% <nickel<3%

manganese<1%

silicon<1%

carbon<0.030%

nitrogen<0.030%

0.075% <titanium<0.20%

0.20% <niobium<0.50%

sulfur<0.01%

phosphorus<0. 1%

the balance being mostly or wholly iron and impurities resulting fromthe smelting of the materials necessary for the production.

Preferably, the steel is defined in its composition by weight asfollows:

22% <chromium<27%

1% <molybdenum <3%

1% <nickel<3%

0.3% <manganese<0.5%

0.3% <silicon<0.5%

carbon<0.030%

nitrogen<0.030%

0.075% <titanium<0.20%

0.20% <niobium<0.50%

aluminum<0.05%

sulfur<0.01%

phosphorus<0.1%

the balance being mostly or wholly iron and impurities resulting fromthe smelting of the materials necessary for the production.

The other characteristics of the invention are:

the elements of the composition satisfy the following relationship:

0.07% ≦ΔNb=% Nb+7/4% Ti-7(% C+% N)≦0.4%.

the composition furthermore contains less than 0.20% of copper.

The description which follows, and the appended figures, all given byway of non-limiting example, will make the invention clearly understood.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 shows three ductile-brittle transition curves for a steel ofcomposition A (11721) according to the invention.

FIG. 2 shows two ductile-brittle transition curves for a steel ofcomposition B (11722) according to the invention.

FIG. 3 shows ductile-brittle transition curves after rapid cooling ofthe hot-rolled steel strip.

FIG. 4 shows ductile-brittle transition curves as a function of varyingcontents of nickel and molybdenum.

FIG. 5 shows comparative pitting-corrosion test curves.

The group of steels having more than 18% of chromium includes steelswhich are difficult to convert because of the high proportion ofchromium which they contain. However, the high chromium contents havethe effect of increasing the corrosion resistance, compared withso-called 17% chromium ferritic steels.

Aluminum and zirconium, introduced into the composition in residualamounts, are contained in a proportion of impurities due to theproduction.

Copper typically cannot be introduced in a smaller amount since it iscontained in the composition of the base materials used for productionof the steel.

Molybdenum improves the resistance to generalized corrosion in acidmedium and to pitting corrosion. However, it is preferably limited inconcentration in order to avoid problems in the area of hot fracturetoughness.

Nickel improves the resistance to corrosion in acid medium, but amaximum limit is preferably imposed since too great an amount of nickelembrittles the steel.

The steel according to the invention, preferably in the form of slab,undergoes a particular heat treatment in order to reduce itsembrittlement, especially when the steel is highly stabilized. This isbecause it has been observed that uncontrolled cooling of the steelduring its conversion produces embrittlement of the said steel.

In a preferred aspect of to the invention, a slab of the steel issubjected to through-cooling at a rate of between 400° and 600° C./hourdown to a temperature of 900° C. Next, the slab is subjected to rapidthrough-cooling at a rate of between 1200° and 1400° C./hour, forexample by immersing the slab in a pool of water until this reaches atemperature of approximately 550° C.

EXAMPLES

Three types of cooling were tested and compared, by applying the processto a slab highly stabilized with niobium and titanium, with ΔNb equal to0.33.

In the heat treatment, the slab is subjected to cooling in a pool for aperiod of less than 10 min. Before entering the pool at a temperature ofapproximately 900° C., the slab is cooled through at a rate of about600° C./h, and then at a rate of 1300° C./h on going into the pool downto at least a temperature of approximately 550° C.

The chemical compositions of steels A (11721) and B (11722) according tothe invention are given in Table 1.

                                      TABLE 1                                     __________________________________________________________________________    Chemical compositions of the steels                                                   Composition                                                           Steels  C  Si Mn Ni  Cr Mo Cu S  Al Ti Nb O.sub.2                                                                          N.sub.2                                                                          ΔNb                     __________________________________________________________________________    Ref 316 L                                                                             .017                                                                             .588                                                                             1.636                                                                            11.51                                                                             17.65                                                                            2.15                                                                             .056                                                                             .0032                                                                            .003                                                                             .004                                                                             .004  .030                             Ref. F18MT                                                                            .010                                                                             .351                                                                             .401                                                                             .233                                                                              17.96                                                                            2.109                                                                            .007                                                                             .0012                                                                            .041                                                                             .071                                                                             .375                                                                             19/23                                                                            .019                                                                             .296                          Steel A (11721)                                                                       .017                                                                             .347                                                                             .391                                                                             2.032                                                                             22.79                                                                            2.015                                                                            .011                                                                             .0017                                                                            .005                                                                             .113                                                                             .374                                                                             32/32                                                                            .018                                                                             .327                          Steel B (11722)                                                                       .018                                                                             .368                                                                             .397                                                                             1.98                                                                              23.00                                                                            2.021                                                                            .010                                                                             .0021                                                                            <.002                                                                            .110                                                                             .373                                                                             33/36                                                                            .017                                                                             .320                          Steel C (11519)                                                                       .017                                                                             .322                                                                             .405                                                                             2.05                                                                              23.08                                                                            2.02                                                                             .121                                                                             .0053                                                                            .025                                                                             .117                                                                             .440                                                                             43/42                                                                            .015                                                                             .421                          Steel D (11694)                                                                       .027                                                                             .307                                                                             .419                                                                             2.04                                                                              23.22                                                                            2.10                                                                             .010                                                                             .0016                                                                            .035                                                                             .049                                                                             .300                                                                             25/29                                                                            .022                                                                             .043                          Steel E (11605)                                                                       .016                                                                             .404                                                                             .406                                                                             1.99                                                                              23.12                                                                            1.94                                                                             .010                                                                             .0011                                                                            .033                                                                             .099                                                                             .352                                                                             29/36                                                                            .015                                                                             .308                          Steel F (11606)                                                                       .017                                                                             .313                                                                             .409                                                                             1.97                                                                              23.09                                                                            1.93                                                                             .009                                                                             .0019                                                                            .048                                                                             .072                                                                             .250                                                                             27/32                                                                            .020                                                                             .117                          __________________________________________________________________________

FIG. 1 shows three ductile-brittle transition curves for steel A(11721). Curves 1 and 2 are the ductile-brittle transitioncharacteristics of steel A, according to the invention, this steelhaving been rapidly cooled in the pool for 10 to 5 min, respectively.Curve 3 shows the brittle-ductile transition characteristic of steel A,this steel not having been rapidly cooled.

Curve 2 shows a transition temperature at 140° C. and relatively highhot fracture toughness values at a temperature of between 190° C. and360° C., while without cooling, as shown by Curve 3, the steel remainsbrittle with a transition temperature of 296° C. and a low hot fracturetoughness, that is to say approximately 80 J/cm², at a temperature of350° C.

The fact of increasing the time spent in the pool improves the fracturetoughness characteristics little. With 10 minutes spent in the pool, atransition temperature of 113° C. and hot fracture toughness valuesgreater than only approximately 30% are obtained. In addition, thetemperature of the slab on leaving the pool is lower, which can causeproblems, for example when grinding the slab.

The cooling according to the invention avoids the precipitation ofembrittling intermetallic compounds of the Mo-enriched Fe₂ Nb type.

FIG. 2 shows two characteristic ductile-brittle transition curves forsteel B (11722) compared with a fracture toughness characteristic ofsteel A. It will be observed that the cooling gives a ductile-brittletransition temperature of 124° C. and hot fracture toughness values attemperatures of between 180° C. and 260° C. of about 160 J/cm².

These values show that steel B according to the invention has improvedcharacteristics compared with steel A, this being explained by the factthat steel A is less stabilized. In fact, the composition of the steel Asatisfies the relationship: ΔNb=0.32%.

According to the invention, after the slab has been hot rolled, thestrip obtained is subjected to rapid cooling and is then coiled at atemperature of less than 600° C., preferably at a temperature close to550° C.

Tests were carried out using steel C (11519) whose composition is givenin Table 1. This steel is highly stabilized.

The fracture toughness characteristics shown in FIG. 3 relating to thesteel according to the invention are compared with a reference steel ofthe F18MT type, a 17% chromium steel, which has not undergone rapidcooling.

A very marked improvement resulting from the rapid cooling of thehot-rolled strip is observed. The transition temperature moves, fromapproximately 220° C., to 172° C. for rapid cooling and coiling at 600°C. and to 147° C. for coiling at 550° C. It may be noted that Curve 1,which represents steel C (11519) subjected to rapid cooling and coilingat 550° C., is similar to the characteristic of the reference steel. Thesame applies to Curve 2 which represents the characteristic of steel C(11519) subjected to rapid cooling and coiling at 600° C., Curve 3 beinga comparative curve of a characteristic of steel C according to theinvention, but which has not been subjected to rapid cooling.

The heat treatment according to the invention makes it possible toobtain, for a steel containing more than 18% chromium, characteristicscomparable to those of so-called 17% chromium steels. It substantiallyimproves its fracture toughness properties, especially by lowering theductile-brittle transition temperatures.

The carbon and nitrogen contents of the steel according to the inventionare limited in order to reduce the intergranular corrosion phenomena.

It has been observed that the nickel and molybdenum contents must belimited.

FIG. 4 shows a characteristic ductile-brittle transition curve of asteel C according to the invention containing 2% molybdenum and 2%nickel, this characteristic being, on the one hand, compared with thatof a steel of the same general composition and containing 3.2%molybdenum and 2% nickel, and, on the other hand, with that of a steelof the same general composition and containing 2% molybdenum and 4%nickel.

Comparison of these three curves shows that it is necessary, accordingto the invention, to limit the molybdenum and nickel contents to a valueof less than 3%.

From the corrosion standpoint, it is necessary to define the minimumcontents of the stabilizing elements titanium and niobium in order toensure intergranular corrosion resistance. As previously, therelationship:

ΔNb=% Nb+7/4×% Ti-7×(% C+% N) corresponds to the excess of stabilizersafter the carbides and nitrides have precipitated.

The intergranular corrosion resistance is evaluated by the Strauss testapplied to specimens on which a line of TIG melting has been traced.

The tested specimens of steel D (11694) satisfying the relationship ΔNbequal to 0.043 showed no cracking.

Likewise, on more stabilized steels, such as steel E (11605) and steel F(11606) for example, it is observed that there is no disbandment afterthe Strauss test. At greater levels of stabilization, for example ΔNbgreater than 0.1, there is no loosening, while at the stabilizationlevel of steel D this is observed, without thereby leading to theappearance of cracks. The value of ΔNb equal to 0.043 is thereforereally a minimum level to ensure intergranular corrosion resistance,below which cracks will occur.

FIG. 5 shows pitting corrosion characteristics on polished specimens,aged in air and then subjected to polarization with a 100 mV min⁻¹ scan,in a 0.5M aqueous sodium chloride solution having a pH equal to 6.6 anda temperature of 70° C.

The various characteristics shown in the figure indicate that steels Eand F have greater pitting corrosion resistance than steels taken as areference, such as 316 L and F 18 MT steels.

From the standpoint of crevice corrosion, steel C (11519) and steel D(11694) have been compared with a 316 L reference steel. Steel C hastitanium and niobium contents higher than steel D. These elements appearto have no appreciable influence on the crevice corrosion behavior ofthe steel.

This comparison was made on polished specimens, aged in air and thensubjected to polarization at a potential of -750 mV/SCE for 2 minfollowed by holding at a floating potential for 15 min. The specimensare then subjected to a 10 mV.min⁻¹ scan between -750 mV/SCE and 1000mV/SCE, the specimens being immersed in a 2M aqueous sodium chloridesolution having a pH of 1.0 and 1.5.

The table below collates, for the steels tested, the values of thepotentials and current densities corresponding to the activity peaksmeasured on the polarization curves in a 2M NaCl solution.

    ______________________________________                                        pH = 1.0            pH = 1.5                                                  I(μA/cm.sup.2)                                                                          E(mV/SCE)  I(μA/cm.sup.2)                                                                       E(mV/SCE)                                   ______________________________________                                        316 L  70        -335       15      -370                                      Steel C                                                                              91        -474       1.5     -340                                      Steel D                                                                              47        -478       1.0     -338                                      ______________________________________                                    

These results show that steel D, less stabilized than steel C from thestandpoint of the titanium and niobium concentration, behaves in thesame way as the said steel C. The activity peaks occur at the samepotential and have a maximum intensity of the same order of magnitude.

It will be noted that the variations in the titanium and niobiumcontents do not alter the crevice corrosion behavior of the steelsaccording to the invention.

In general, a value of ΔNb equal to 0.040% is regarded as a minimumvalue in order to ensure intergranular corrosion resistance.

As a titanium content greater than 0.075% is fixed by the requirementsfor pitting corrosion resistance, the minimum niobium content istherefore preferably greater than 0.30%.

French patent application 96 03258 is incorporated herein by reference.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:
 1. A process for producing a ferritic stainlesssteel, wherein steel having a composition comprising, by weight based ontotal weight:18% <chromium<27% 1% <molybdenum<3% 1% <nickel<3%manganese<1% silicon<1% carbon<0.030% nitrogen<0.03 0% 0.075%<titanium<0.20% 0.20% <niobium<0.50% sulfur<0.01% phosphorus<0.1%ironand impurities resulting from smelting materials necessary forproduction of said steel, is subjected, in a first phase, to coolingfrom a temperature above 950° C. at a rate of between 400° C. and 600°C./hour down to a temperature of 950°-850° C. and then, in a secondphase, to rapid cooling at a rate of between 1200° C. and 140° C./h to atemperature of from 550° C.-650° C.
 2. The process as claimed in claim1, wherein the steel is subjected to hot rolling after the first phasecooling but before being subjected to rapid cooling and is then coiledat a temperature of less than 600° C.
 3. The process as claimed in claim1, wherein the steel is in slab form, and its composition comprises byweight based on total weight:22% <chromium<27% 1% <molybdenum<3% 1%<nickel<3% manganese<1% silicon<1% carbon<0.030% nitrogen<0.030% 0.075%<titanium<0.20% 0.20% <niobium<0.50% sulfur<0.01%. phosphorus<0.1%. 4.The process as claimed in claim 1, wherein the steel further comprises,in its composition by weight, less than 0.20% of copper.
 5. The processas claimed in claim 1, wherein the elements of the composition of thesteel satisfy the following relationship:0.07% <ΔNb=% Nb+7/4% Ti-7(% C+%N)<0.4%.
 6. A ferritic stainless steel obtained by the process asclaimed in claim
 1. 7. A ferritic stainless steel obtained by theprocess as claimed in claim
 3. 8. The steel as claimed in claim 6,wherein the elements of the steel composition satisfy the followingrelationship:0.07% <ΔNb=% Nb+7/4% Ti-7(% C+% N)<0.4%.
 9. The steel asclaimed in claim 6, wherein the steel composition furthermore containsless than 0.20% of copper.
 10. The steel as claimed in claim 7, whereinthe elements of the steel composition satisfy the followingrelationship:0.07% <ΔNb=% Nb+7/4% Ti-7(% C+% N)<0.4%.
 11. The steel asclaimed in claim 7, wherein the steel composition furthermore containsless than 0.20% of copper.
 12. The process of claim 1, wherein saidsteel is at a temperature of from 1200°-1300° C. when first phasecooling is initiated.